(WJLA) - The failure of a jack assembly led to the collapse of a portion of a Montgomery County mall parking garage that killed one worker and seriously injured another, a report on the incident concludes.

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In an executive summary on the May 23 incident at the Westfield Montgomery Mall in Bethesda, investigators say that that the scaffold legs of a jacking tower at the construction site where the accident happened were not being used properly, causing it and a portion of the garage to come down.

According to the investigation, which calls the incident a "tragic construction incident," a safety tower at the garage began to twist and fail. At that point, workers jacked up a tee in an effort to remove the beam.

At that point, though, investigators say that the scaffold legs of the jacking tower were extended beyond manufacturers recommendations; at the same time, its extension sections were not braced properly. Those two missteps reduced the load capacity of the jack, causing it to fail.